1. Field of the Invention
The present invention relates to an antenna system that can be installed in a vehicle, such as an automobile.
2. Description of the Related Art
As a countermeasure technique against reception disturbance that can be caused by noise that is emitted from a noise source in a vehicle, Patent Document 1 (Japanese Unexamined Patent Publication No. 2006-295797) has been known, for example. Patent Document 1 discloses a technique against the reception disturbance of a television broadcast signal in a VHF band or in a UHF band.
Whereas, in so called a “hybrid vehicle” that combines an engine and a motor, or in an electric vehicle, electronic devices are installed, such as a motor and an inverter. The noise that is emitted from such an electronic device may affect a radio wave in a Medium Frequency (MF) band or in a Low Frequency (LF) band, so that receiving performance of an antenna for receiving a radio wave in the MF band or the LF band may be lowered.
In the technique of Patent Document 1, however, two antenna elements are required to be disposed, so that a difference between respective differences from a noise source to the two antenna elements is equal to a half wavelength of the noise. Thus, it is required to identify a noise source, and a situation may not be handled where there are multiple noise sources. Furthermore, a wavelength of a radio wave in the MF band or the LF band is significantly greater than that of a radio wave in the VHF band or the UHF band. Consequently, for a case of using it as a countermeasure against the reception disturbance of a radio wave in the MF band or the LF band, one of the antenna elements may not be disposed in the vehicle because it may be necessary to dispose the antenna elements so that they are greatly separated from each other. Hence, the reception disturbance in the MF band or in the LF band may not be handled with the technique of Patent Document 1.
Accordingly, there is a need for an antenna system that can suppress degradation of the reception performance of receiving a radio wave in the MF band or in the LF band, due to noise that is emitted from inside the vehicle.